Jonathan P. Hosler, PhD

Background

Graduated in 1983 - University of Michigan

Research interests

Energy transducing proteins of mitochondria

Humans derive the bulk of their energy from mitochondrial oxidative phosphorylation (oxphos). In the "oxidative" part of this process, high energy electrons from foodstuffs are transferred to oxygen. The energy of electron transfer is used to pump protons across the inner mitochondrial membrane, creating a voltage gradient. In the 'phosphorylation' part of oxphos, the ATP synthase of mitochondria uses the energy of the voltage gradient to generate ATP. The last of the three proton pumps of oxphos is cytochrome c oxidase (CcO), a complex of integral membrane proteins that reduces oxygen to water and uses the free energy of this reaction to pump protons. The heme a3-CuB oxygen reduction site of CcO is located within the transmembrane region of the protein complex and electrons, protons and oxygen move through specific pathways to reach this buried site. Two redox cofactors, CuA and heme a, transfer the electrons that soluble cytochrome c delivers to the outer surface of the complex. Two proton uptake pathways, the D and K paths, transfer protons to the heme a3-CuB center from the inner surface of CcO. Oxygen moves in through a pathway that begins in the lipid bilayer. The exit pathway for pumped protons, and for the water generated at the active site, remain to be clearly defined.

For much of our work, my colleagues and I use molecular methods to create new forms of the aa3-type CcO of the α-proteobacterium Rhodobacter sphaeroides. As an extant relative of the bacterium that gave rise to mitochondria, R. sphaeroides synthesizes a CcO that is closely related to the core subunits of mitochondrial CcO. This CcO is a mechanistic model of mammalian CcO and crystal structures can be obtained. Engineered forms of R. sphaeroides CcO are analyzed by biochemical and biophysical methods. The current aims of our research are to:

Use cytochrome c oxidase to study the roles of specifically bound phospholipids that are consistently found to be structural elements of integral membrane proteins. "Structural" lipids have been identified in crystal structures of bacterial and bovine CcO. Of particular interest to us are two structural lipids in sub-unit III of CcO whose binding sites are conserved from R. sphaeroides to humans.

Use CcO to study how proteins create pathways for long-distance proton transfer. We are working to elucidate how the protein surface controls the entry of protons into the D and K pathways that lead to the buried active site. We are also working to identify residues involved in the transfer of protons from the pumping site to the outer surface of the protein.

Elucidate the mechanism of 'suicide' inactivation of CcO. Our previous efforts have demonstrated that a side-reaction at the heme a3-CuB center may occur during any catalytic cycle to cause the irreversible loss of activity for that CcO molecule. Current models of suicide inactivation include oxidative damage by oxygen reduction intermediates and/or the disassociation of CuB. Strong determinants of the probability of the inactivation event are the rate of proton uptake into the D pathway and the structural integrity of the lipids bound within sub-unit III.

Use CcO to study the mechanisms used by copper chaperones and other assembly proteins to assemble a metalloprotein with multiple metal centers. We have shown that a specific membrane-bound copper chaperone (Cox11p) is required for the assembly of CuB, and we are now working to identify residues of Cox11p that mediate its interaction with CcO and residues essential for binding copper. More recently, we have found that the assembly protein Surf1p, also a membrane protein, is required for the efficient assembly of the heme a3-CuB center. Both Cox11p and Surf1p are required for the assembly of human CcO and mutations lead to disease. R. sphaeroides CcO is particularly useful for studies of metal center assembly since, in contrast to mitochondrial CcO, partially assembled CcO forms that lack individual metal centers can be isolated and characterized.